Apparatus for electrostatically depositing a uniform coating of finely divided coating material on moving substrates

ABSTRACT

Apparatus for applying powder on continuously moving strip, particularly metal powder on continuously moving metallic strip, by introducing a cloud of powder into an electrostatic deposition zone through which the strip passes, the cloud of powder being introduced by a plurality of nozzles which have a specific relation to each other and to the width of the strip to obtain substantially uniform deposition of powder transversely and longitudinally of the strip.

United States Patent [1 1 Austin et al.

.APPARATUS FOR ELECTROSTATICALLY DEPOSITING A UNIFORM COATING OF FINELY DIVIDED COATING MATERIAL ON MOVING SUBSTRATES Inventors: Lowell W. Austin; James N. Baker, both of Weirton, W. Va.

National Steel Corporation, Pittsburgh, Pa.

Filed: Aug. 15, 1973 Appl. No.: 388,476

Assignee:

U.S. Cl 118/621, 118/627, 118/629, 118/634 Int. Cl. B05c 5/02 Field of Search 118/621, 627, 629, 634, 118/635, 638; 239/3, 15

References Cited UNITED STATES PATENTS 6/1948 Ransburg 118/634 X 1 Feb. 18, 1975 3,558,052 1/1971 Dunn 239/3 3,575,138 4/1971 Austin et al. .6 118/634 3,621,815 1l/1971 Walbe r 239/15 3,745,034 7/1973 Smith ct al 118/627 X Primary E.raminerMervin Stein Assistant Examiner-Leo Millsteim [57] ABSTRACT Apparatus for applying powder on continuously moving strip, particularly metal powder on continuously moving metallic strip, by introducing a cloud of powder intoan electrostatic deposition zone through which the strip passes, the cloud of powder being introduced by a plurality of nozzles which have a specific relation to each other and to the width of the strip to obtain substantially uniform deposition of powder transversely and longitudinally of the strip.

' 4 Claims, 12 Drawing Figures PATENTEU FEB l 8i975 sum 1 or 5 F'ATENTEU 81975 SHEET 2 OF 5 pp; L

PATENTED 1 8% 3, 866.571

' SHEET 3 OF 5 COATING WEIGHT (gm/ff comma WEIGHT (gm m COATING WEIGHT (gm/H 30 ACROSS STRIP (in) l APPARATUS FOR ELECTROSTATICALLY DEPOSITING A UNIFORM COATING OF FINELY DIVIDED COATING MATERIAL ON MOVING SUBSTRATES BACKGROUND OF THE INVENTION This invention relates to an apparatus for electrostatically depositing powder on continuously moving strip, particularly metal powder on continuously moving metallic strip.

Apparatus has been provided in the past for electrostatically depositing metal powder on continuously moving metallic strip. For example, US. Pat. No. 3,575,138 provides an elongated chamber through which metallic strip is continuously moved and a single nozzle for introducing a cloud of metal powder into the chamber where the particles of metal powder are charged and moved toward and onto the strip under the influence of an electrostatic field. Because of the characteristics of flow of an aerosol of metal powder and compressed gas such as air to and through a nozzle and the characteristics of the discharge from the nozzle, apparatus employing a single nozzle for introducing a cloud of metal powder into an electrostatic deposition zone are not capable of electrostatically depositing metal powder with sufficient uniformity transversely and longitudinally of the metallic strip when the width of the strip exceeds 10 inches, particularly when the width of the metallic strip is 20 to 40 inches.

It is accordingly an object of the present invention to provide an apparatus for electrostatically depositing metal powder on continuously moving metallic strip of a width greater than 10 inches, particularly strip having widths of 20 to 40 inches, in which a substantially uniform coating of metallic powder is deposited on the strip, transversely and longitudinally, for a wide range of strip speeds and metal powder coating weights.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings, in which similar reference characters denote similar elements throughout the several views:

FIG. 1 is a diagrammatic view in side elevation and partially in section of a coating apparatus constructed in accordance with the principles of the present invention;

FIG. 2 is a diagrammatic view in plan and partially in section of the coating apparatus shown in FIG. 1;

FIG. 3 is an enlarged view partly in section of a part of the apparatus shown in FIGS. 1 and 2;

FIG. 4 is an enlarged view in plan and partly in sec tion of a portion of the apparatus shown in FIG. 1;

FIG. 5 is a view in section taken along the line 5-5 of FIG. 4;

FIG. 6 is a view in section taken along the line 66 of FIG. 5;

FIG. 7 is a graph showing the uniformity of coating weight on metal strip obtained when practicing the present invention;

FIG. 8 is another graph showing the uniformity of coating weight on metal strip obtained when practicing the present invention;

FIG. 9 is a further graph showing the uniformity of coating weight on metal strip obtained when practicing the present invention;

FIG. 10 is a chart illustrating measured coating weights of deposited metal powder when practicing the invention according to Example I;

FIG. 11 is a chart illustrating measured coating weights of deposited metal powder when practicing the invention according to Example II; and

FIG. 12 is a chart illustrating measured coating weights of deposited metal powder when practicing the invention according to Example Ill.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIGS. 1 and 2 of the drawings disclose apparatus embodying the principles of the present invention for applying a uniform coating of metal powder on continuously moving metallic strip with means for wetting the strip prior to the metal powder deposition and for heat treatment of the strip having the metal powder deposited thereon. It is to be understood that the principles of the present invention may be embodied in dissimilar apparatus which provides for different treatment of the strip before and after the metal powder is deposited.

As shown in FIGS. 1 and 2, the apparatus includes an uncoiling device 10 supporting a coil 11 of metallic strip 12 such as steel strip. The strip leaves the coil 11 and moves through a strip wetting device 13 which functions to controllabl y wet the top surface of the strip for metal powder adherence to the strip. The wetted strip then moves through an electrostatic coating zone 14 where a coating of metal powder is deposited on the top surfaceof the strip, as viewed in the drawing.

The strip with deposited metal powder then moves through a heat treating furnace 15 and then to a coiling device 17 for coiling the coatedstrip in coil 18. The uncoiling device 10 and the coiling device 17 may be of conventional construction capable of moving the strip through the apparatus at a controlled speed and under required tension. The heat treating furnace 15 also may be of conventional construction such as an infrared furnace.

The electrostatic coating zone 114 is defined by-an elongated housing 19 constructed of electrically insulating material and having a strip entry opening 20 and a strip exit opening 21. Means are provided for charging particles of metal powder within the zone 14 and for producing an electrostatic field within the zone 14 for moving charged particles of metal powder in a direction toward the top surface 22v of the strip. Such means is shown in the form ofa plurality of wires 23 extending transversely of the strip 12, spaced from each other in the direction of movement of the strip and disposed in a plane spaced from and parallel to the top surface 22 of the strip. The wires are so supported by structure 24 of non-insulating material located within the housing 19. The wires 23 are continuously charged with a potential of one polarity, such as positive, from power source 25 and the strip 12 is maintained at the opposite potential such as by connection 26 to ground. Excess metal powder that may accumulate within the housing 14 downstream of the electrostatic coating zone 14 in the direction of strip movement may be removed by conduits 27 which communicate with a vacuum source and metal powder separator-collector 28. Also, conduits 29 connected to a source of vacuum, not shown, extending transversely of the strip and located above and below the strip and having openings facing the strip, collect excess powder and minimize the flow of metal powder through the exit end 21.

In accordance with the principles of the present invention, a cloud of metal powder is introduced into the coating zone 14 by means of a plurality of discharge nozzles located at the entrance end of the housing 19. As described in detail below, the discharge nozzles are so constructed and are so positioned relative to each other and to the strip so that a cloud of metal powder, that is of substantially uniform density as to the metal powder transversely and longitudinally of the strip, is continuously maintained in the coating zone 14 so as to obtain under the influence of the electrostatic field a continuous application of metal powder to the surface 22 of the strip that is of substantially uniform weight transversely and longitudinally of the strip. Four discharge nozzles 30, 31, 32, and 33 are shown; however, it is to be expressly understood that the principles of the present invention may be employed using any number of discharge nozzles greater than two depending upon variables including the width of the strip being processed. Each of the discharge nozzles is fed with a pressurized aerosol composed of metal powder and air or an inert gas produced by a separate blower, such as blowers 34, 35, 36, and 37, respectively, through a pipe section 38 and a tapered transition section 39. The blowers may be of any conventional construction capable of providing the flow rates and velocities required and each is driven by a variable speed motor 40. The inlet to each of the blowers 36, 37, 38, and 39 is continuously fed with measured quantities of metal powder from hoppers 41, 42, 43, and 44 and associated conduits 45, 46, 47, and 48, respectively, as well as an inert gas such as air which may also be controlled. A suitable arrangement for this purpose is shown in FIG. 3 in relation with blower 36 and hopper 41. As shown, the conduit 45 extends through the inlet opening 49 of the blower 36 and terminates in communication with the cavity 50 of the blower and a metal powder feed screw 51 is located within the conduit 45 and in communication with the powder in the hopper 41 to transfer metal powder to within the cavity 50 of the blower. The feed screw 51 is driven by a variable speed motor 52 to continuously feed controlled amounts of metal powder into the blower. The inlet opening 49 may be shrouded by ductwork 53 which communicates with a gas inlet conduit 54 provided with an adjustable valve 55 to control the mass of gas entering the blower.

As shown more clearly in FIGS. 4, 5 and 6, the nozzles 30, 31, 32, and 33 each include spaced planar sidewalls which convergingly taper in a longitudinal direction and a planar top portion 61 and a spaced planar corresponding bottom portion, not shown, both of which divergingly taper in a longitudinal direction to provide an elongated rectangular or slit-shaped discharge opening 62 the area of which corresponds to the area of the opening in the feed conduit 38, the taper of the side walls and the top and bottom portions being in the direction of strip movement. The nozzles 30, 31, 32, and 33 are supported by a transverse member 63, having vertical supports not shown, with their discharge openings 62 facing in a direction toward the charging wires 23 and lying in a common plane perpendicular to the direction of movement of the strip 12 and located within the housing 19 inwardly of the inlet 20. Also, the nozzles are mounted with the long dimension of the slit-shaped discharge opening 62 lying in a common plane parallel to and spaced above the path of the strip 12. The nozzles 30, 31, 32, and 33are symmetrically positioned relative to the width of the strip 12 and adjacent nozzles are equally spaced transversely of the direction of strip movement. Preferably, a portion of the discharge opening of the outboard nozzle 30 extends transversely beyond the edge 64 of the strip and a portion of the discharge opening of the other outboard nozzle 33 extends transversely beyond the other edge 65 of the strip.

It has been discovered that in order to continuously provide a cloud of powdered metal in the electrostatic coating zone 14 so characterized that the metal powder electrostatically deposited on the strip will be of substantially uniform weight transversely and longitudinally of the strip, certain relationships are required between the width of the strip and the transverse spacing of adjacent nozzles and between the width of the strip and the long dimension of the slit-type discharge openings. in particular, the relationships are: (l) the long dimension of the slit-type discharge openings should not be greater than one-fourth the width of the strip, (2) the space between nozzles should not be greater than one-half the long dimension of the slit-type discharge openings, and (3) the sum of (i) the combined long dimension of the slit-type discharge openings of all of the nozzles and (ii) the total spacing between adjacent nozzles, should not be less than thewidth of the strip. It has been discovered that satisfaction of these relationships makes possible the electrostatic deposition of metal powder substantially uniformly on metallic strip throughout ranges of coating weights and strip speeds. These relationships have also been found to be applicable to nozzles tapered no greater than 15 which is the maximum taper that precludes metal powder sticking to and accumulating on the inside planar surfaces of the nozzles thus making it possible to obtain the desired uniformity of metal powder deposition with nozzles of reasonable length.

When a pressurized aerosol of metal powder and gas is fed to a nozzle that is divergingly tapered to provide a slit-type discharge-opening, the discharge from the nozzle, as viewed in a plane parallel to the long dimension of the discharge opening, will diverge upon the front of the discharge from the nozzle moving away from the discharge opening as represented by the broken lines a and b associated with the nozzles 30, 31, 32, and 33 of F IG. 4. The front of the discharge, that is, the discharge as viewed in a plane parallel to the plane of the discharge opening, immediately upon leaving the discharge opening of the nozzle will include a center portion, extending equally from both sides of a plane perpendicular to the long dimension of the discharge opening and passing through its mid-point, in which the density of metal powder will be substantially unifonn, and side portions extending with decreasing metal powder density to both ends of the discharge opening. As the front of the discharge moves away from the discharge opening, the center portion of substantially uniform metal powder density will comprise a progressively decreasing percentage and the side portions of outwardly varying metal powder density will comprise a progressively increasing percentage of the total transverse dimension of the discharge. In addition, with a nozzle having a discharge opening of a greater transverse dimension and with the other dimension being smaller, the center portion of the discharge of substantially uniform metal powder density comprises a relatively less percentage of the transverse dimension of the discharge when the front is at the discharge opening and such percentage decreases as the front moves away from the discharge opening. Notwithstanding the foregoing, the present invention obtains a cloud of metal powder in the electrostatic deposition zone, of substantially uniform metal powder density, which extends transversely of the strip throughout its width as well as an appreciable distance in the direction of strip movement. This is accomplished in accordance with the present invention by the use of stationary nozzles in specific relation to each other and to the strip material. The relationship is such that the patterns of the discharge from the nozzles, as represented by lines a and b of FIG. 4, overlap between adjacent nozzles in a region beyond the plane of the discharge openings 62 which extends a substantial distance in the zone 14 in the direction of movement of the strip so that the discharges of all of the nozzles present a combined front of substantially uniform metal powder density extending transversely longitudinally of the strip. The outboard nozzles 30 and 31 are preferably positioned so that their discharge openings extend outwardly beyond the edges 64 and 65 of the strip, respectively, to aid in obtaining uniform metal powder density in the regions near the marginal edge portions of the strip. The retaining action of the sidewalls of the housing 19 influence the uniformity of metal powder distribution in the outermost extremities of the combined front. The positioning of the sidewalls of the housing and the outboard nozzles relative to the edges of the strip may be utilized in combination to obtain uniform metal powder distribution in the outer transverse portions of the combined cloud for a wide range of coating weights and strip speeds with a high efficiency of metal powder utilizatron.

An experimental apparatus built in accordance with the principles of the present invention included a belt made of galvanized steel strip having a width of 42 inches and a thickness of 0.22 inch. The belt was mounted with respect to the housing 19 so that the upper reach of the belt moved through the housing in a direction from its entry end 20 to its exit end 21 to simulate continuously moving strip. Four discharge nozzles were positioned in the entry 20 of the housing 19 above the upper reach of the belt as shown in FIGS. 4, 5 and 6. The discharge opening of each nozzle had a long dimension transversely of the direction of movement of the belt of 6.inches and a depth dimension of 4 inch and adjacent nozzles, in the plane of their discharge openings, were spaced from each other by a distance of 2-% inches. Each nozzle was fed with an aerosol of metal powder'and compressed air delivered by a blower driven by a 34 horsepower, 3,500 rpm. motor. Metal powder consisting of --200 mesh Fe-Cr powder having about 85 percent Cr was controllably fed to the inlet of the blowers by a mechanism similar to that shown in FIG. 3. The experimental apparatus was operated at simulated strip speeds of 10, 30 and 60 feet per minute with the rate of powder feed to the blowers adjusted to obtain a coating weight of IS grams per square foot. The uniformity of the metal powder coating transversely of the belt was within gm/fi square. FIG. 7 illustrates a typical metal powder distribution obtained. The values of coating weight which the curve of FIG. 7 is based on were obtained by removing and then weighing the powder deposited on incremental areas of the belt extending in side-by-side relation transversely of the direction of movement of the belt. Also, the experimental apparatus was operated at simulated strip speeds of 10, 30 and 60 feet per minute with the rate of powder feed to the blowers adjusted to obtain a coating weight of 30 grams per square foot. The metal powder coating obtained had a uniformity transversely of the belt essentially within a range of :4 grams per square foot of the desired coating weight of 30 grams per square foot. FIG. 8 illustrates a typical metal powder distribution obtained. After the experimental apparatus was calibrated as to the quantity of metal powder introduced into the: deposition zone for different settings of powder feed rate and blower speed and after the metal powder utilization efficiency was obtained, the experimental apparatus was adjusted to obtain a coating weight of 28 gm/ft squared at a line speed of 25 feet per minute. The powder distribution of the coating obtained is depicted in FIG. 9 of the drawings.

An apparatus embodying the principles of the present invention for applying metal powder to continuous metallic strip was constructed in the manner shown in FIGS. 1 and 2. The nozzles were spaced from each other transversely of the strip material by 2% inches and the discharge opening of the nozzles had a dimension of 6 inches in a transverse direction and a depth of inch. Each of the nozzles was fed with an aerosol of metallic powder and compressed air from separate blowers to which metallic powder was fed by an apparatus similar to that shown in FIG. 3. The impeller of the blowers had a small diameter of5 inches and a large diameter of 10 inches and each blower was driven by a horsepower motor at 3,500 r.p.m. Such apparatus was operated in accordance with the following examples.

EXAMPLE I A coil of low carbon aluminum killed strip steel having a width of 30 inches and a thickness of 0.036 inch and a total weight of about 9,000 pounds, previously cleaned in a conventional manner, was placed on the uncoiler l0 and threaded through. the apparatus past the wetting device 13, through the coating zone 14 and the furnace 15 to the coiling device: 17 and then moved through the line at a speed of 15 feet per minute. The wetting device wetted the top surface of the strip with a ferrous chloride solution, a charging voltage of about 20,000 volts was applied to the charging wires within the coating zone and the furnace 15 was operated at a temperature of 200F.-250F. The motors driving the blowers were set at about of maximum output and the powder feed motor controls were set to feed metal powder into the blowers at a rate of about 460 grams per minute in order to obtain a coating of about 35 g/ft at a strip speed of 15 feet per minute in view of the metal powder utilization efficiency of the electrostatic coating process determined from previous operations. The metal powder consisted of -2(l0 mesh Fe-Cr powder having 73% Cr.

After a portion of the strip had passed through the coating zone, the line was stopped and the weight of the coating on the top surface of the strip was measured. This was accomplished by collecting powder deposited on the strip in ten discrete 9 square inch areas and then weighing the collected powder. The discrete areas were located in consecutive side-by-side relation in a 3-inch wide portion of the strip surface extending across the strip perpendicular to the direction of movement of the strip as shown in FIG. A wherein, as viewed in that Figure, the area on the right was at the edge of the strip adjacent the nozzle 30 and the area on the left was at the other edge of the strip adjacent the nozzle 33. The weight of powder collected from each area is indicated on FIG. 10A in grams per square foot. Thereupon the apparatus was operated to move the strip through the line at a speed of feet per minute until prior to the rear end of the coil passing from the uncoiler when the apparatus was again stopped and the weight of the metallic powder coating on the strip again measured. This was accomplished by collecting metallic powder depos ited in five discrete l8-square inch areas extending transversely of the strip as shown in FIG. 10B and weighing the powder collected. The weight of powder collected in each area is indicated on FIG. 10B in grams per square foot. As seen from FIG. 108 the average weight of the coating across the strip within the 3- inch portion of the strip surface was 36.44 g/ft and the coating as measured did not vary from the average beyond i4 g/ft The coated coil of steel strip was subsequently subjected to a chromizing process and the chromized strip was found to have a substantially uniform iron-chromium alloy coating.

EXAMPLE II A coil of low carbon aluminum killed strip steel having a width of 30 inches and a thickness of 0.036 inch with a total weight of about 9,000 pounds, previously cleaned in a conventional manner, was placed on the uncoiler 10 and threaded through the apparatus past the wetting device 13, through the coating zone 14 and the furnace 15 to the coiling device 17 and then moved through the line at a speed of 15 feet per minute. The wetting device wetted the top surface of the strip with a ferrous chloride solution and a charging voltage of about 20,000 volts was applied to the charging wires within the coating zone, and the furnace 15 was operated at a temperature of 200F.250F. The motors driving the blowers were set at about 80% of maximum output and the powder feed motor controls were set at values appropriate for obtaining a coating of 35 g/ft at a strip speed of 15 feet per minute as determined from previous operations of the apparatus. The metal powder consisted of 200 mesh Fe-Cr powder having 73% Cr.

After a portion of the strip had passed through the coating zone, the line was stopped and the weight of the coating on the top surface of the strip was measured. This was accomplished by collecting powder deposited on the strip in 10 discrete 9 square inch areas and then weighing the collected powder as described in Example I. The weight of the powder collected from each area is shown on FIG. 11A in grams per square foot. In view of the non-uniformity of the powder weights found in the discrete areas as shown in FIG. 11A, the powder feed rate to nozzle 31 was slightly increased and the powder feed rate to nozzle 33 was slightly decreased and the apparatus was thereafter operated to move the strip through the line at a speed of 15 feet per minute and again stopped and the weight of coating in the discrete areas was again collected and weighed to obtain the weights in grams per square foot as shown in FIG. 11B. Thereupon the apparatus was again operated to move the strip through the line at a speed of 15 feet per minute until prior to the end of the coil passing from the uncoiler the apparatus was again stopped and the weight of the metallic powder coating within 18 inch square areas of the strip was measured as described in Example I. The weight of powder collected on each area is shown on FIG. 11C in grams per square foot. The average weight of the coating across thestrip within the areas measured was 34.35 g/ft and the coating as measured did not vary from the average beyond :4 g/ft The coated coil of steel strip wassubsequently subjected to a chromizing process and the chromized strip was found to have a substantially uniform ironchromium alloy coating.

EXAMPLE III A coil of columbium treated low carbon strip'steel having a width of 32 inches and a thickness of 0.048 inch with a total weight of about 9,000 pounds, previously cleaned in a conventional manner, was placed on the uncoiler l0 and threaded through the apparatus past the wetting device 13, through the coating zone 14 and the furnace 15 to the coiling device 17 and then moved through the line at a speed of 15 feet per minute. The wetting device wetted the top surface of the strip with a ferrous chloride solution and a charging voltage of about 20,000 volts was applied to the charging wires within the coating zone, and the furnace 15 was operated at a temperature of 200F.-250F. The motors driving the blowers were set at about of maximum output and the powder feed motor controls were set at values appropriate for obtaining a coating of .35 g/ft at a strip speed of 15 feet per minute as determined from previous operations of the apparatus. The metal powder consisted of 200 mesh Fe-Cr powder having 73% Cr.

After a portion of the strip had passed through the coating zone, the line was stopped and the weight of the coating on the top surface of the strip was measured. This was accomplished by collecting powder deposited on the strip in eleven discrete areas and then weighing the collected powder as described in Example I. The weight of powder collected from each area is shown in FIG. 12A in grams per square foot. In view of the nonuniformity of the weight of the powder in the discrete areas as shown in FIG. 11A, the rate of powder fed to the nozzle 30 was slightly increased and the rate of powder fed to the nozzles 32 and 33 was slightly decreased, and thereafter the apparatus was operated to move the strip through the line at a speed of 15 feet per minute until prior to the rear end of the coil passing from the uncoiler, the apparatus was again stopped and the weight of the metallic powder coating on the strip again measured in the manner described in Example I. The weight of powder collected in each of the areas is shown on FIG. 128 in grams per square foot. The average weight of the coating across the strip within the 33- inch band was 35.7 g/ft and the coating as measured did not vary from the average beyond :4 g/ft The coated coil of steel strip was subsequently subjected to a chromizing process and the chromized strip was found to have a substantially uniform iron-chromium alloy coating.

The principles of the present invention may be used to apply any powder to any strip material providing the powder and the strip material are so characterized so that the powder may be electrostatically deposited on the strip material. The powder may be of any size providing the particle size of the powder permits the formation of an aerosol of the powder with compressed gas and provides a cloud of the powder in the deposition zone. Thus, the principles of the present invention may be used to apply a wide variety of metal powder on metallic strip and is not limited to the depositing of Fe-Cr powder on steel strip as described herein. Furthermore, the principles of the present invention may be used to deposit metal powder on both sides of metallic strip such as by providing similar discharge nozzles on the underside of the strip. In addition, the features of the present invention may be used to uniformly deposit metal powder on metallic strip which is thereafter processed in a manner different from the process described for producing a different product.

Accordingly, it is to be understood that the foregoing description including the specific examples is for the purpose of description only and not as a definition of the limits of the invention, reference for the latter purpose being had to the appended claims.

What is claimed is: 1. Apparatus for applying a coating of metal on metallic strip comprising an elongated housing of nonelectrically conducting material having an entrance opening and an exit opening and defining an elongated passageway extending between the entrance opening and the exit opening, means for continuously moving metallic strip through the elongated passageway of the housing along a path in a direction from the entrance opening to the exit opening in out-of-contact relation with the material of the housing, means located in the housing for charging metal particles introduced into the pssageway of the housing and for producing an electrostatic field in the passageway for moving charged metal particles in a direction toward the strip passing through the passageway, and means for introducing a cloud of metal powde. into the passageway at its entrance opening generally in the direction of movement of the strip through the passageway, the last-named means including a plurality of fixed nozzles spatially positioned transversely of the path of movement of the strip through the elongated passageway, and means for feeding to each of the nozzles a pressurized aerosol including a gas and metal powder, each nozzle having a discharge opening in the form of a slit, each nozzle having side walls tapering outwardly in the direction of strip movement, the angle of taper of the nozzles being no greater than 15, the nozzles being positioned with the long dimension of the slit discharge openings lying in a common plane parallel to and spaced from the path of the strip material and with the discharge opening lying in a common plane perpendicular to the path of the strip material, the long dimension of the slit discharge opening of each nozzle being no greater than one-fourth the width of the strip material. 2. Apparatus for applying a coating of metal on metallic strip as defined in claim 1 in which the slit discharge openings of the nozzles are similar and have a long dimension about twenty times its short dimension. 3. Apparatus for applying a coating of metal on metallic strip as defined in claim 1 in which the nozzles are spaced from each other transversely of the strip in the: plane of the slit discharge openings by a distance no greater than onehalf the long dimension of the slit discharge openings. 4. Apparatus for applying a coating of metal on metallic strip as defined in claim 3 in which the summation of (i) the product of the number of nozzles and the long dimension of the slit discharge openings of the nozzles and (ii) the product of the number of nozzles less one and the space between adjacent nozzles, is not less than the width of the strip material. 

1. Apparatus for applying a coating of metal on metallic strip comprising an elongated housing of nonelectrically conducting material having an entrance opening and an exit opening and defining an elongated passageway extending between the entrance opening and the exit opening, means for continuously moving metallic strip through the elongated passageway of the housing along a path in a direction from the entrance opening to the exit opening in out-of-contact relation with the material of the housing, means located in the housing for charging metal particles introduced into the pssageway of the housing and for producing an electrostatic field in the passageway for moving charged metal particles in a direction toward the strip passing through the passageway, and means for introducing a cloud of metal powder into the passageway at its entrance opening generally in the direction of movement of the strip through the passageway, the last-named means including a plurality of fixed nozzles spatially positioned transversely of the path of movement of the strip through the elongated passageway, and means for feeding to each of the nozzles a pressurized aerosol including a gas and metal powder, each nozzle having a discharge opening in the form of a slit, each nozzle having side walls tapering outwardly in the direction of strip movement, the angle of taper of the nozzles being no greater than 15*, the nozzles being positioned with the long dimension of the slit discharge openings lying in a common plane parallel to and spaced from the path of the strip material and with the discharge opening lying in a common plane perpendicular to the path of the strip material, the long dimension of the slit discharge opening of each nozzle being no greater than one-fourth the wiDth of the strip material.
 2. Apparatus for applying a coating of metal on metallic strip as defined in claim 1 in which the slit discharge openings of the nozzles are similar and have a long dimension about twenty times its short dimension.
 3. Apparatus for applying a coating of metal on metallic strip as defined in claim 1 in which the nozzles are spaced from each other transversely of the strip in the plane of the slit discharge openings by a distance no greater than one-half the long dimension of the slit discharge openings.
 4. Apparatus for applying a coating of metal on metallic strip as defined in claim 3 in which the summation of (i) the product of the number of nozzles and the long dimension of the slit discharge openings of the nozzles and (ii) the product of the number of nozzles less one and the space between adjacent nozzles, is not less than the width of the strip material. 